This invention relates to a plastic working method and its apparatus.
Forging is a typical method for plastic working. However, ordinary forging requires an extremely large load, since an entire active surface of a shaping tool (die) is in surface pressure contact with an overall surface of the workpiece. In order to avoid this deficiency, rotational forging has been developed since it requires a relatively small load for shaping the workpiece. According to the rotational forging, a part of the active surface of the shaping tool is in pressure contact with a part of the surface of the workpiece, and such partial forging is repeatedly applied to the workpiece, so that entire surface of the workpiece undergoes forging.
A conventional apparatus for plastic working such as coining or sizing has an upper die and a lower die. A workpiece is mounted on the lower die and is depressed between the upper and lower dies. The upper die is movable up and down with respect to the lower die along a predetermined axis or is swung (two dimensional swinging motion) about a single point on the workpiece by means of the operation of an eccentric cam, a crank, a screw mechanism or a toggle mechanism.
According to another type of a prior art plastic working apparatus, a die is swivelled (three dimensional motion) at a predetermined slant angle with respect to an axis passing through a predetermined position of a workpiece, and in accordance with the swivelling motion, the plastic work is applied to the workpiece in comformance with the shape of a shaping die portion of the die. In this type, one of the dies, such as an upper die, is adapted to perform a circular rocking motion relative to the other die (lower die).
However, according to the conventional apparatus, since the load center of the upper die relative to the workpiece is always kept constant, the plastic flow of the workpiece material is restrained. It is therefore difficult to perform the plastic working with high accuracy.
Further, according to another type of the conventional apparatus, since the swivelling center of the die is always kept constant, it is impossible to increase an "expansion amount" of the workpiece material, and excellent swaging may not be attainable. In other words, plasticized metal flow is not sufficiently reached into the finely configured portion of the workpiece so as to manufacture a product having a complicated outer shape. Thus, it would be difficult to perform the work with high precision. In addition, it would be impossible to apply the working operation to a complicated work since the operational mode is limited in such prior art apparatus.
Such conventional forging apparatus are described in, for example, U.S. Pat. No. 3,990,285, and G. B. Patent Nos. 319,065, 1,205,171, 1,224,259 and 1,225,260.